When converting PVC, i.e. polyvinyl chloride, the resin ordinarily undergoes a fluxing step after which it is processed to its desired shape. It is, of course, necessary that the resin remain stable at the high temperatures which are encountered during these processing and fluxing procedures. Additionally, the resin must yield under stress. It must have adequate flow properties and it must lend itself to millling and extrusion. Since unmodified PVC does not always possess all of these properties, it has been necessary or desirable to add various processing aids to the resin. For instance, acrylic ester polymers in grannular form are dry-mixed with the PVC resin and then milled on a calender or extruded. Alternatively, it is suggested by British Pat. Nos. 1,062,308 and1,015,334 that the vinyl chloride may be polymerized in the presence of the pre-formed acrylic ester polymer or the acrylic ester monomer may be added to the polyvinyl chloride latex and polymerized in situ. The resulting product is a emulsion grade polyvinyl chloride blended with the polyacrylate ester modifier. Such modified polymers and processing aids permit faster calendering with improved gloss, better surface qualities on extrusion, freedom from plating and other benefits.
In the above identified related abandoned patent application No. 760, 067 there is disclosed a process for preparing processing aids for PVC by means of a process involving initiating the suspension polymerication of PVC, revolving unreacted vinyl chloride monomer from the system after polymerization is at least 60% complete, introducing methyl methacrylate monomer into the system whereupon the polymerization is resumed and continued until the methyl methacrylate is polymerized.
However, since filing the latter patent application, it has been found that the products resulting from the polymerization process disclosed therein are somewhat deficient with respect to their melt flow properties, i.e., their fluidity characteristics, during subsequent processing operations such as molding.
Still another significant deficiency of the process disclosed in the above identified abandoned application is the absence of any means of controlling the size of the initially prepared PVC particles as well as of the particles of the polyacrylate-modified PVC derived therefrom. There was, moreover, no recognition given to the necessity for producing the desired processing aid in a certain particle size range. For example, if the processing aid particles are too large in size, their use in certain applications, such for example as in bottle blowing, will result in the preparation of products having a grain-like surface or so-called "applesauce" effect.
Thus, it is the prime object of this invention to provide an improved method for preparing a polyacrylate-modified PVC processing aid. Still another object involves providing an efficient means for controlling both the size of the PVC particles prior to their modification with the subsequently added MMA, as well as the size of the polymerclyate-modified PVC particles derived therefrom. It is a further object to obtain these polyacrylate-modified PVC processing aids by means of a process which is at all times fully reproducible with respect to the properties inherent in the products derived therefrom. Various other objects and advantages of this invention will be apparent upon reading the disclosure which follows hereinafter.